1. Field of the Invention
The present invention relates to an optical unit for use in image forming apparatuses such as a laser beam printer and a digital copying machine.
2. Description of the Related Art
Hitherto, a scanning optical unit has been used in image forming apparatuses such as a laser beam printer and a digital copying machine. One example of the image forming apparatuses will be described below with reference to FIG. 9.
FIG. 9 is a schematic sectional view of a color image forming apparatus for recording image information in respective colors on a plurality of photoconductive drums and forming a color image by using a plurality of scanning optical units.
The color image forming apparatus comprises four scanning optical units 100, four photoconductive drums 151-154, four development units 131-134, and four transfer units 121-124, the last threes corresponding to the four scanning optical units in one-to-one relation. The color image forming apparatus further comprises a fusing unit 140 and a feed belt 141.
The four scanning optical units 100 are disposed corresponding to respective colors of C (cyan), M (magenta), Y (yellow) and Bk (black), and irradiate scanning lights onto the photoconductive drums 151, 152, 153 and 154 in accordance with respective image signals, thereby forming latent images on the drum surfaces. The latent images are developed by the development units 131-134 using toners in the respective colors, and the toner images on the photoconductive drums 151-154 are transferred respectively by the transfer units 121-124 to a recording material, e.g., a sheet of paper, which is transported on the feed belt 141. The not-yet fused toners on the sheet of paper are then fused and fixed to the sheet of paper under heating by the fusing unit 140.
In each of the scanning optical units, a light beam emitted from a light source and optically modulated in accordance with the image signal is cyclically deflected to perform scanning by an optical deflecting unit, e.g., a rotating polygon mirror. The deflected scanning light beam is focused by an image-forming optical system having an fθ characteristic into a spot-like shape on an image-formed surface of the photoconductive drum. The spot on the image-formed surface forms the electrostatic latent image with a combination of main scan by the optical deflecting unit and sub-scan by rotation of the photoconductive drum, thereby recording an image.
A scanning optical unit discussed in Japanese Patent Laid-Open No. 2004-054019 will be described below with reference to FIG. 7. FIG. 7 is a perspective view, partly broken, of the discussed scanning optical unit.
In the scanning optical unit 100, as illustrated in FIG. 7, a laser beam emitted from a light source 101 becomes a substantially parallel light beam through a collimator lens 102. After the quantity of light has been adjusted by a diaphragm 103, the parallel light beam enters a cylindrical lens 104 having refractive power only in the direction of sub-scan.
The parallel light beam having entered the cylindrical lens 104 exits the same lens in such a state that the beam remains substantially parallel in a main scan cross-section, while it is converged only in a sub-scan cross-section. The exiting beam is focused to form a linear image on a reflecting surface 105a of a rotating polygon mirror 105 that serves as an example of a deflection scanning unit.
The light beam deflected to perform scanning by the rotating polygon mirror 105 (i.e., the scanning light), is focused on an image-formed surface of a photoconductive drum 120 through an fθ lens 106 which constitutes an image-forming optical element having an of characteristic.
A dot image (spot) formed on the image-formed surface of the photoconductive drum 20 scans the drum surface in the direction of an arrow B while the rotating polygon mirror 105 is rotated in the direction of an arrow A. By such a combination of main scan and sub-scan with the rotation of the photoconductive drum 120 about its own rotation axis, an image can be recorded on the photoconductive drum 120 that serves as a recording medium.
If the surfaces of optical parts, e.g., the fθ lens 106 (made up of 106a and 106b), and the reflecting surface 105a of the rotating polygon mirror 105 become dirty, the image quality is deteriorated e.g., due to a reduction in the quantity of light. To avoid that deterioration, as illustrated in FIGS. 8A and 8B, the scanning optical unit is enclosed by a cover 150 and a sealing member 151 such that dust, will not enter an inner space of the scanning optical unit from the exterior.
As illustrated in FIG. 7, the collimator lens 102, the cylindrical lens 104, the rotating polygon mirror 105, and the fθ lens 106 are all disposed inside an optical box 152. The cover 150 is operatively connected to the optical box 152 by screws 153 so as to cover an opening of the optical box 152, whereby the inner space of the optical box is enclosed.
Positive sealing of the optical box 152 is facilitated by arranging the sealing member 151 in a properly compressed state between the fθ lens 106b and the cover 150. The sealing member 151 is given as an elastic member, e.g., an elastic soft urethane foam.
In the above-described scanning optical unit, a scanning line may not be curved in the direction of sub-scan. To that end, a transparent plane-parallel plate is disposed between the image-forming lens (fθ lens) and the photoconductive drum and is rotated about its longitudinal axis to compensate for a curving of the scanning line. With regards to a biased magnification of the scanning line, it is also known to make compensation by moving the plane-parallel plate in the longitudinal direction.
Such a registration adjustment is performed within a range of several tens microns, and an equivalent level of high accuracy can also be useful for an amount of compensation to be made using the image-forming lens.
However, even when the shape of the scanning line in each scanning optical unit is compensated for by using the plane-parallel plate, the relative positional relationship among the optical parts can be changed after the compensation due to, e.g., a deformation of the optical box containing the optical parts, and the scanning line can be deformed or deviated in shape or position sometimes.
One example of the deformation of the optical box is as follows.
In the scanning optical unit illustrated in FIGS. 8A and 8B, when the cover 150 and the optical box 152 made of materials having different linear expansion coefficients from each other are fixed by the screws 153 at plural points, a distortion occurs with temperature changes, and the optical box 152 can be deformed to cause the above-mentioned deviation of the scanning line as illustrated in FIGS. 10A and 10B.
In order to avoid such a deformation of the optical box with temperature changes, it was conventionally discussed to fix the cover and the optical box made of materials having different linear expansion coefficients from each other by screwing at one point and snap-fitting at plural points as illustrated in FIGS. 11 and 12. This conventional approach is effective in reducing the deformation due to heat and lessening the influence upon the scanning line.
In a scanning optical unit shown in FIGS. 11 and 12, a laser beam L1 generated from a light source unit 201 is condensed by a cylindrical lens 202 into a linear shape on a reflecting surface of a rotating polygon mirror 203 mounted to a deflecting unit 204. With rotation of the rotating polygon mirror 203, the laser beam is deflected to perform scanning.
The deflected scanning laser beam enters an fθ lens made up of a refractive optical element 206 and a diffractive optical element 207, and is then focused to form an image on a photoconductive member of a photoconductive drum 120 serving as an image carrier.
An optical box 208 containing the above-mentioned various optical parts is in the form of a resin molding. A cover 210 made of a metal sheet is mounted to the optical box 208 in such a state that an elastic member (sealing member) 209, e.g., a urethane foam, is disposed on an upper surface of the diffractive optical element 207. An inner space of the optical box 208 is thereby enclosed.
The cover 210 is operatively connected to the optical box 208 in one fixed portion 250 by screwing and at a plurality of fixed portions 251a-251c by snap-fitting.
The cover 210 is mounted to the optical box 208 by sliding the cover 210 in the direction of an arrow 252 to be engaged with the optical box 208 in the snap-fitted portion 251a. 
For example, where the cover 210 and the optical box 208 are subjected to thermal expansion with a rise in temperature, the cover 210 and the optical box 208 are fastened together only in one fixedly screwed portion 250. On the other hand, in the snap-fitted portion 251c, the cover 210 is operatively connected to the optical box 208 while facilitating deformation of the cover 210 in the sliding direction (i.e., in the direction of the arrow 252) due to thermal expansion. This is similarly applied to the snap-fitted portions 251a and 251b. It is therefore possible to absorb deformations caused due to the difference in thermal expansion coefficient between the cover 210 and the optical box 208.
Another example of the deformation of the optical box is as follows.
FIGS. 13A and 13B are sectional views showing another known scanning optical unit. In this known scanning optical unit, a laser beam generated from a semiconductor laser (not shown), which is disposed inside an optical box 301, becomes a parallel beam through a collimator lens. After the parallel beam is shaped by an aperture, the shaped light beam is condensed by a cylindrical lens 302 onto a reflecting surface of a rotating polygon mirror 303. With rotation of the rotating polygon mirror 303, the light beam is deflected to perform scanning in the direction perpendicular to a rotation axis of the rotating polygon mirror 303 (i.e., in the direction of main scan), and it is focused to form an image on a photoconductive member of a rotating drum D through an fθ lens 304 and a folding mirror 305. The light beam focused on the photoconductive member forms an electrostatic latent image by a combination of a main scan with the rotation of the rotating polygon mirror 303 and a sub-scan with the rotation of the rotating drum D.
In order to obtain a highly-accurate image in that image forming apparatus, one can adjust a spot size of the optical beam focused on the photoconductive member of the rotating drum D. To that end, the cylindrical lens 302 is moved along the optical path of the light beam by using a tool, and the cylindrical lens 302 is operatively connected to the optical box 301 by bonding or another suitable manner at such a position that the spot size on the photoconductive member has a proper value.
Also, a deviation and an inclination of the scanning line formed by the light beam on the photoconductive member have to be adjusted so that the image is formed at a proper position e.g., with respect to a sheet of recording paper. To that end, when the optical box 301 is operatively connected to an optical bench, first and second positioning members 312 are adjusted for proper positioning of the optical box 301 relative to the rotating drum D.
In the above-described scanning optical unit, if the cover 320 closing an upper opening of the optical box 301 has a deformation, e.g., a warp, as illustrated in FIG. 13A, the optical box 301 is entirely deformed following the warp of the cover 320, as illustrated in FIG. 13B, when the cover 320 is fastened to the optical box 310 by screws, for example. This gives rise to the problem that the spot size on the photoconductive member of the rotating drum D is changed, or that the scanning line is deviated.
As one solution to cope with such a deformation caused by mounting of the cover, Japanese Patent Laid-Open No. 2000-010035 discusses that openings for adjustment work are formed in the cover, as illustrated in FIG. 14, thus facilitating the adjustment work to be performed, as required, after the cover has been mounted to the optical box.
As in the case of FIG. 14, the semiconductor lens, the collimator lens, the cylindrical lens 302, the rotating polygon mirror 303, the fθ lens 304, and the folding mirror 305 can be arranged inside an optical box 401, and an upper opening of the optical box 401 is enclosed by a cover 420.
The cover 420 has an opening 421 opened to face the cylindrical lens 302 and an opening 422 opened to face the positioning member 312. The tool used for the adjustment work can be inserted into the optical box 401 through each of the openings 421 and 422. The cover 420 is operatively connected to the optical box 401 by not-shown fixing members, e.g., screws.
The spot size of the light beam focused on the photoconductive member of the rotating drum D is adjusted by changing the position at which the cylindrical lens 302 is assembled to the optical box 401, after the mounting of the cover 420. For example, the tool can be inserted through the opening 421 of the cover 420 to move the cylindrical lens 302 along the optical path of the light beam, and the cylindrical lens 302 is operatively connected to a bottom wall of the optical box 401 at such a position that the spot size on the photoconductive member has a proper value. Also, the deviation and the inclination of the scanning line on the photoconductive member have to be adjusted by changing the position of the positioning member 312.
The adjustment work for the deviation and the inclination of the scanning line is performed by inserting the tool (not shown) through the opening 422 of the cover 420. After the adjustment work, each positioning member 312, 412 is operatively connected to the optical box 401 by the known manner, and the optical box 401 is operatively connected to the optical bench 430 by a screw 431. The openings 421 and 422 are closed by a cover sheet 423 after the adjustment work.
Thus, by inserting the tool through the opening 421 or 422 of the cover 420 and adjusting the position of the cylindrical lens or the positioning member after the cover 420 has been fixed to the optical box 401, even if the cover 420 is warped and the optical box 401 is deformed when the cover is fastened to the optical box, the resulting distortion can be compensated for with the adjustment of the cylindrical lens and the positioning member.
Further, Japanese Patent Laid-Open No. 2000-171744 discusses a method of fixing a lid of an optical box to the optical box at three points by screwing.
In addition to the deformation of the optical box caused by the difference in linear expansion coefficient between the components and the deformation of the optical box caused by the warp of the cover, however, there are other factors causing the deviation of the scanning line on the photoconductive member.
The other factors causing the deviation of the scanning line include a deformation of the optical box generated when the optical box is mounted to the image forming apparatus, and vibration generated by the image forming apparatus.
In many recent scanning optical units, as illustrated in FIGS. 15 and 16, the unit size and cost are reduced by scanning light beams from a plurality of light sources over a plurality of photoconductive members with one deflecting unit. Particularly in that type of scanning optical unit, the deformation of the optical box can be suppressed more strictly.
For that purpose, it is useful to increase the rigidity of the scanning optical unit in its entirety and to restrict the deformation of the optical box with higher reliability. Because of a limitation in restricting the deformation of the optical box just by increasing the rigidity thereof, it is further required to more firmly mount a cover having higher rigidity and to increase the rigidity of the entire optical box including the cover.
Among the above-cited Patent Documents intended to cope with the deviation of the scanning line on the photoconductive member, in Japanese Patent Laid-Open No. 2004-054019, the cover is fastened to the optical box at one point by screwing. Therefore, the rigidity of the scanning optical unit is hardly increased even with the mounting of the cover to the optical box. Also, in Japanese Patent Laid-Open No. 2000-171744, because the lid is operatively connected to the optical box at three points just by screwing, high rigidity cannot be obtained in the entirety of the optical box.
FIGS. 17 and 18 are an overall perspective view and a plan view of another known scanning optical unit, respectively. As illustrated in FIG. 17, a scanning optical unit 30 comprises an optical box 31 and a cover 32. Inside the optical box 31, as illustrated in FIG. 16, there are disposed a deflecting unit 4, fθ lenses 5a and 5b, and a plurality of folding mirrors 6, 7 and 8 for introducing a plurality of light beams, which are deflected to perform scanning by the deflecting unit 4, to a plurality of corresponding photoconductive members. Also, the cover 32 has a plurality of slits 32a through which the plurality of light beams deflected to perform scanning are facilitated to exit toward the exterior of the scanning optical unit. Further, a plurality of reinforcing ribs 32b are provided near the slits 32a, respectively, to increase the rigidity of an entire box structure in cooperation with side walls disposed along an outer periphery of the optical box.
In addition, to increase the rigidity of the entire optical box when the cover and a box frame are fixed together, the cover 32 is fastened to the optical box 31 at six points indicated by A-1 to A-6, as illustrated in FIG. 17.
Because of variations in production accuracy of parts such as the cover and the box frame, however, flatness of an opening plane cannot be held at a satisfactory level depending on the variations in parts production accuracy when the parts are fastened together at many points in the same plane as in the above-mentioned known scanning optical unit. Consequently, the box frame is also affected and a torsion occurs in the optical box.